A novel two-dimensional NMR relaxometry pulse sequence for petrophysical characterization of shale at low field

Low field two-dimensional nuclear magnetic resonance (2D-NMR) relaxometry is a powerful probe for the characterization of heterogenous, porous media and provides geometrical, physical and chemical information about samples at a molecular level and has been widely used in shale studies. However, NMR signals of shale decay so rapidly, dry sample for particular, that the conventional two-dimensional pulse sequence is either not sensitive enough to short relaxation components or takes too much measurement time. In this paper, 2D-NMR relaxometry correlation based on partial inversion recovery CPMG (PIR-CPMG) pulse sequence is proposed and illustrated to improve the contrast over saturation recovery CPMG (SR-CPMG) and reduces the T₁ encoding time of inversion recovery CPMG (IR-CPMG) for petrophysical characterization of shale. Subsequently, the kernel function and inversion method of this sequence are presented and the reliability of the inversion method is testified by numerical simulation. Next, theoretical analysis is conducted to validate the advantages of PIR-CPMG. Ultimately, experiments on copper sulfate solution, artificial sandstone, and shale samples are performed, respectively, to verify the feasibility and effectiveness of the proposed pulse sequence. The results demonstrate that the PIR-CPMG sequence is time-saving and high-contrast, especially for the short relaxation components. This pulse sequence can be utilized in bench-top NMR core analyzer and downhole well logging, potentially, to achieve integrated petrophysical characterization of shale.